1.Preheat oven to 180°C (160°C fan forced). Grease a deep 22cm (9-inch) round cake pan, line base and side with baking paper.
2.Blend cocoa with the water in a small bowl, cool. Sift flour and sugar into a large bowl, add cocoa mixture, egg yolks, oil and extract. Beat with an electric mixer until smooth and mixture is changed to a paler colour.
3.Beat egg whites in a large bowl with an electric mixer until soft peaks form, fold into cocoa mixture, in four batches. Pour mixture into pan.
4.Bake cake about 1 hour. Stand in pan 5 minutes, turn, top-side up, onto a wire rack to cool.
5.Make walnut praline, Place sugar in a heavy-based frying pan; cook over heat, without stirring, until sugar is melted and golden brown. Add nuts, pour onto a greased oven tray. Cool. Blend or process praline with chocolate until finely chopped.
6.Next make brandied buttercream. Beat butter in a small bowl with an electric mixer until as white as possible, beat in sifted icing sugar and cocoa, then brandy.
7.Split cold cake into three layers, join layers with some of the buttercream. Spread top and sides of cake evenly with remaining buttercream. Decorate with walnut praline. Serve immediately.
To decorate the side of the cake with praline (as pictured), spread just the side of the filled cake with buttercream. Hold the top and the bottom of the cake in your hands and roll the cake like a wheel in the praline. Place the cake on a serving plate, then spread the top with remaining buttercream. This cake is best made on the day of serving.